1. Technical Field
The present invention relates generally to test equipment used to test printed circuit boards and more specifically to the use of a wireless fixture in a printed circuit board test system.
2. Discussion of the Prior Art
After printed circuit boards (PCB""s) or other electrical interconnect planes or devices have been manufactured and loaded with components, and before they can be used or placed into assembled products, they should be tested to verify that all required electrical connections have been properly completed and that all necessary electrical components have been attached or mounted to the board in proper position and with proper orientation. Other reasons for testing printed circuit boards are to determine and verify whether the proper components have been used and whether they are of the proper value. It is also necessary to determine whether each component performs properly (i.e., in accordance with the specification). Some electrical components and electro-mechanical components also may require adjustment after installation.
Loaded-board testing has complex multiplexed tester resources and is capable of probing soldered leads, vias and testpads on loaded boards with topside and bottom side components. Loaded-board testing includes analog and digital tests, such as tests for electrical connectivity, voltage, resistance, capacitance, inductance, circuit function, device function, polarity, vector testing, vectorless testing, and circuit functional testing. Loaded-board testing requires very low contact resistance between the test targets and the fixture components.
Advances in circuit board and electronic component packaging technology have escalated the probe spacing demands placed on loaded-board test equipment. Existing state-of-the-art technology requires loaded-board test equipment capable of accessing test targets which are spaced apart by 50 mils (center to center) or less, where test targets are physical features on a PCB or electronic component which may be probed during testing. One of the greatest challenges faced by loaded-board test equipment manufacturers now and in the future is a high false failure and test malfunction rate caused by physical and electrical contact problems. These problems are exacerbated by existing fixture limitations in probing accuracy, probing pitch (center to center spacing), and surface contamination.
As component and board geometries shrink and become denser, loaded-board testing becomes more difficult using standard fixtures. Existing shortwire, loaded-board fixtures can consistently hit test targets equal to or greater than 35 mils in diameter with equal to or greater than 75-mil pitch. Targets that are smaller or more closely spaced cannot be probed with consistency due to prohibitive component and system tolerance stack-ups.
A variety of test fixtures have heretofore been available for testing loaded boards on test equipment. A device under test (DUT) often embodies a PCB loaded with electronic components and electronic hardware. One approach that addresses the limited-access problems during testing is the use of wireless fixtures. The term xe2x80x9climited-accessxe2x80x9d refers to something that cannot easily be reached, or accessed, due to physical restrictions or constraints. For example, a limited-access PCB may contain many targets that are too closely spaced to accurately probe using existing fixture technology.
The term xe2x80x9cstandard-accessxe2x80x9d refers to that which can be reached, or accessed, using existing fixture technology. The use of wireless fixtures allows a greater coupling of test targets to test resources. FIG. 1 shows one wireless fixture that attempts to address limited-access problems during testing. The wireless fixture of FIG. 1 shows a fragmentary cross-sectional view of a representative wireless fixture that is currently used in the Agilent 3070 test system. The PCB 120 provides electrical connections between pins 100 and double-ended sockets 20. The PCB 120 also has holes for mounting screws 80. This PCB 120 has permanently attached pins 100. The test system interface probe 90 provides the interface between the PCB 120 and the test system. The test system interface probe 90 allows the pin 100 to provide an electrical interface between the PCB 120 and the test system. The alignment plate 110 is used to orient the pin 100 properly with respect to the test system interface. The double-ended socket 20 is a receptacle for a standard spring probe 30 used to make electrical contact to the DUT 130. The double-ended socket 20 is press mounted into the probe mounting plate 10 and has a spring loaded probe tip that makes electrical contact to PCB 120. Probe mounting plate 10 is attached to the fixture frame 40, which is coupled to alignment plate 110. A guide plate 70, comprising a thin insulating sheet, is used to protect the double-ended socket 20 during assembly. The guide plate 70 is oriented relative to the probe mounting plate 10 and the PCB 120 by the use of a standoff 50 between the guide plate 70 and the probe mounting plate 10 and a standoff 60 between the PCB 120 and the guide plate 70. A number of screws 80 are used to resist the spring forces generated by the double end sockets 20 and the test system interface probes 90. These screws 80 pass through the PCB 120, then the first standoff 60, the guide plate 70, the second standoff 50, and finally enter the probe mounting plate 10.
Current wireless fixtures need to assemble the printed circuit board by attaching pins and require the printed circuit board to carry loads imposed by springs used in electrical contacts. Installation and soldering of pins onto a circuit board is not a common process for typical fixture builders. The materials typically used for constructing printed circuit boards are not very stiff. The travel distance for the probe tip in a typical double-ended socket is 0.100 inch so PCB supports must be close together to adequately control deflection of the PCB. In current implementations this leads to the use of a large number of screws and standoffs for mounting the PCB to the probe mounting plate. In order to keep the PCB relatively flat during fixture assembly these mounting screws must be carefully tightened in small increments. This is very time consuming and is a decrement to disassembling the fixture if changes are needed. The standoffs and screws must be located where double ended sockets are not required. Determining an adequate number of mounting screws and locating them must be done before the PCB can be routed which can lead to longer fixture build times. The space needed for the mounting holes in the PCB takes away space for routing traces and can increase the number of layers required which increases the cost of the PCB.
There is a need for an improved wireless test fixture that places less reliance on the printed circuit board or other electrical interconnect device as a structural element, that causes less deflection of the PCB due to contact forces, and that does not require screw holes in the area of double ended sockets. There is a further need for such an improved wireless test fixture that has an assembly process that does not require pins to be attached to the PCB, that allows the PCB to be routed independent of spacer location, that is easier to assemble and disassemble when changes must be made, and that allows faster assembly due to the reduction in the number of screws required. There is a further need in the art for a wireless fixture that allows the reuse of some components when the wireless fixture is retired.
It is an object of the invention to allow routing of the printed circuit board or other electrical interconnect plane in the wireless test fixture independent of spacer location.
It is further an object of the invention that the wireless test fixture use pins separate from the printed circuit board.
It is further an object of the invention that the assembly of the wireless test fixture be faster due to the reduction in the number of screws required.
It is yet another object of the invention that the wireless test fixture assembly be easier to assemble and disassemble when changes must be made.
It is another object of the invention that less reliance be placed on the printed circuit board or other electrical interconnect plane as a structural element.
It is further an object of the invention that some components of the wireless test fixture be reusable.
Therefore, according to the structure of the present invention, a wireless test fixture for the testing of loaded printed circuit boards is disclosed. The wireless test fixture electrically connects one or more test targets on a loaded printed circuit board (device under test) with interface probes of a test system. The test fixture contains a probe mounting plate, constructed from epoxy-glass or similar material. Double-ended sockets for wireless fixtures press-fit into corresponding holes in the probe mounting plate. Spring probes are coupled to the double-ended sockets, and the spring probes are used to establish electrical contact to the device under test. An upper frame is coupled to the probe mounting plate, and the upper frame is able to support the probe mounting plate and provide reference information for placing the upper frame with respect to the electrical interconnect plane in the wireless test fixture. A lower frame is coupled to the upper frame and also coupled to an insulating spacer element, which supports the electrical interconnect plane. The lower frame also provides reference information for locating the upper frame with respect to the electrical interconnect plane and the test system. The electrical interconnect plane is often a printed circuit board, but may also be a printed circuit board that is loaded with electrical and optical components. These components may be used to facilitate the testing process applied to the DUT.
The wireless fixture also contains spacer elements that are able to provide mechanical support and electrical insulation to the electrical interconnect plane. These spacer elements mount between, but are not coupled to, the probe mounting plate and the electrical interconnect plane. Bottom support bars are coupled to the lower frame, and provide mechanical support to the electrical interconnect plane in the wireless test fixture. In one embodiment, the bottom support bars are part of the lower frame. The bottom support bars also support pin carriers that retain and position floating transfer pins that make electrical contact to the electrical interconnect plane. Test system interface pins are spring probes that make electrical contact to the floating transfer pins. The electrical path through the floating transfer pins, the electrical interconnect plane, and the double-ended sockets provide test system access to various test targets of the device under test.